Semiconductor electronics components packaged using chip-on-board (COB) processes often employ a cover element to protect silicon chips from physical damages as well as particle and moisture attacks. This cover element is often bonded onto a circuit substrate using adhesives. The underside of the cover element forms a protective cavity in which the silicon chips are located. To ensure reliability of the package, a strong bonding force between the cover element and the substrate is required throughout the lifetime of the product.
As illustrated in FIG. 1, when such packaging is used for optoelectronics components, the cover element may also serve additional optical functions. For example, lens structures may be formed on the cover element to facilitate optical signal coupling. This additional optical functionality can only be realized if the alignment of the cover element to the optoelectronics chips inside the cavity is precisely maintained (<15 um misalignment) during the assembly process and throughout the lifetime of the product.
Two types of adhesives are conventionally used in bonding the cover element to the substrate: UV-activated epoxy and thermally-activated epoxy. The former provides fast curing time and minimal curing shift while the latter provides stronger bonding strength but requires longer curing time and introduces larger post-curing shift between the two parts.